In the catalytic cracking of hydrocarbons, as catalysts there have usually been employed refractory inorganic oxides such as silica-alumina, silica-magnesia, silica-zirconia, alumina-boria and the like; compositions obtained by dispersing a crystalline aluminosilicate zeolite in these inorganic oxides, and these compositions compounded with clay minerals such as kaolin and the like. Since the catalytic cracking of hydrocarbons is normally carried out for the main purpose of producing gasoline, it is preferable to use catalysts which are high in cracking activity and further can obtain high octane number gasoline fractions with high yields, in other words to use catalysts which are high in gasoline selectivity. In view of this, catalysts have usually been used by preference which are obtained by dispersing a crystalline aluminosilicate zeolite in a siliceous matrix including silica-alumina, silica-magnesia or the like.
It is said that the catalysts for use in catalytic cracking of hydrocarbons preferably should be superior in the thermal and hydrothermal stability and attrition resistance in addition to the above mentioned cracking activity and gasoline selectivity. In the catalytic cracking process of hydrocarbons, it is common that spent catalysts are regenerated by separation of carbonaceous substances deposited thereon, and thereafter said regenerated catalysts are again available for catalytic cracking reaction. Said regeneration treatment comprises stripping hydrocarbons from a spent catalyst with steam, and in succession burning off the carbonaceous deposits from the spent catalyst in the presence of oxygen. Due to this, in case the catalyst is inferior in the thermal and hydrothermal stability, the activity of the catalyst is destroyed at the time of regeneration and the regenerated catalyst exhibits only considerably low cracking activity and gasoline selectivity as compared with fresh catalysts. Further, because of the fact that the recent catalytic cracking, as a rule, mostly uses a fluidized-bed reactor, if the catalyst is insufficient in attrition resistance, the catalyst is pulverized in the fluidized bed and lost out of the system, and this is one reason for damaging the cracking activity and gasoline selectivity of the catalyst, too.
In the light of these circumstances, the inventors of the present application have previously proposed that the catalyst composition obtained by compounding the alumina, which may be detected to be crystalline using the X-ray diffraction method, with the siliceous matrix such as silica-alumina, and further dispersing the crystalline aluminosilicate zeolite therein is superior in cracking activity and gasoline selectivity and further exhibits excellent stability against thermal and hydrothermal and attrition resistance (which see Japanese Laid Open Patent Application No. 152548/1980).
The inventors of the present application have studied to improve the previously presented catalyst furthermore and develop a catalyst for use in catalytic cracking which may exhibit a high selectivity in the preparation of an intermediate fraction such as kerosene, gas oil or the like, and have discovered that a catalyst can be endowed, without damaging the various characteristics of the previously proposed catalyst, with a selectivity against the production of an intermediate fraction, by using a flash calcined alumina referred to afterwards as the alumina which may be detected to be crystalline using the X-ray diffraction method and simultaneously mixing kaolin in the catalyst.